There are several types of devices in which amplification of an applied high frequency signal may be achieved by producing modulation of an electron beam. For example, in a klystron, the signal to be amplified is coupled into an input resonant cavity and produces an electric field which acts on electrons of the beam to modify their velocity and produce bunching. There are usually several subsequent resonant cavities which enable the degree of bunching to be enhanced and a final resonant cavity at which the amplified signal is extracted. After the final cavity, the electrons are directed towards a collector section where they impact on a surface.
Another class of amplifying apparatus, is that known as an inductive output tetrode (IOT), such as a "Klystrode", which is a trade mark of Varian Associates. Such a device employs density modulation of the electron beam and also includes a collector section similar to that used in klystrons.
The electrons which reach the collector are of relatively high energy and their impact on its surfaces tends to result in the production of secondary electrons. The secondary electrons may travel in the opposite direction to the electrons of the beam and may return far enough along the klystron, IOT or other device to interfere with its operation and cause deterioration in performance.
The collector may be operated in what is termed a "depressed" mode, in which it is held at a negative potential in order to improve the operating efficiency of the device. The collector may be of the multistage type, having a number of electrodes which are maintained at respective different negative voltages. However, because the collector is maintained at a negative potential, any secondary electrons which are emitted tend to be accelerated along the tube towards the final resonant cavity.